For manual settings on a DSLR camera you are in full control of your camera's settings including your shutter speed, ISO and aperture.
By setting your camera to manual you have the flexibility to be able to adjust your aperture depending on the lighting conditions and the nature of your shoot i.e. landscape, portraits.
You will find the manual button located at the top of your camera, usually on a dial that you can operate yourself.

Image 1: Manual dial

By using the manual settings you can also adjust your white balance and choose your own depth of field. The lower the aperture, the less depth of field you get.
Also, by setting your camera to manual you can achieve a greater depth of field by manually adjusting your white balance for aperture compensation.

To ensure you get the correct exposure for your lighting conditions it is best to use a light meter. Some cameras have these already built in.
You will find manual settings on all camera devices, including SLR's, DSLR's, phones and compacts.

Image 2: Manual display and controls

Image 3: A page of a camera manual

Image 4: All of the camera's manual dials

Some examples of images shot using the manual mode: